The so-called crown cork caps are well known and widely used, comprising an outer cap member usually made of metal and a sealing insert made of cork or a soft plastic material, whereby upon closing the bottle after filling the lower free edge of the outer cap member is bent against the bottle by means of a flanging press. These known caps have the disadvantage that the closing process is quite delicate and time consuming and that you need a tool to open the bottle, a re-closing of the bottle by means of the same cap being excluded as it usually is more or less deformed.
Other caps are known which are made of a plastic material. Usually, there is used quite a soft plastic material in order to ensure that the cap firmly clings to the edges of the bottle opening and thereby preventing an escaping of liquid or CO.sub.2. A disadvantage of these caps lies in the fact that they become toosoft at higher temperatures and the sealing effect is highly degraded. On the other hand, the pressure in the bottle becomes higher at increasing temperatures and the load onto the cap is higher too. If a plastic material is used, that is harder and thereby more heat-resistant a reliable sealing of the bottle is ensured, but such hard plastic caps are difficult to remove from the bottle by hand, often requiring a tool to open the bottle.